Dynamics of type V menthol-thymol deep eutectic solvents: Do they reveal non-ideality?

TL;DR

This study investigates the molecular dynamics of menthol-thymol deep eutectic solvents, revealing non-ideal mixing behavior, structural rearrangements due to hydrogen bonding, and broad glassforming capabilities with complex relaxation dynamics.

Contribution

It provides the first comprehensive analysis of the dielectric and glassy dynamics of type V menthol-thymol DES, highlighting non-ideality and dynamic heterogeneities.

Findings

Positive deviations from ideal mixing in dielectric strength and glass transition.

Excellent glassforming ability across a wide composition range.

Presence of dynamic heterogeneities and complex relaxation behavior.

Abstract

We have established a comprehensive study of the molecular dynamics of the menthol-thymol mixture, the prototype of the new class of type V deep eutectic solvents. Dielectric spectroscopy and differential scanning calorimetry were combined to assess the dipolar relaxation and glassy dynamics over an extended range of timescales (10-6-10 2 s) and for eleven different compositions including the two pure constituents. Positive deviations from ideal mixing approximation were demonstrated for both the solvent dielectric strength and the glass transition. They support the idea that preferential H-bond interactions between unlike molecules induce structural rearrangements in the DES mixtures. Excellent glassforming capability of the DES mixtures was demonstrated for a broad range of compositions (xThymol = 0.4-0.7). In this case, the dipolar dynamics could be assessed from the normal liquid to the deeply supercooled state. Many salient parameters of the relaxation functions, including fragility, non-Debye character and rotation-translation decoupling point to the development of dynamic heterogeneities.